Income Taxes (Schedule Of Changes To Unrecognized Tax Benefits And Related Interest) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Mar. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	$ 156	[1]	$ 148	[1]	$ 246	[1]	$ 48
Increases based on tax positions	7	[1]	5	[1]	22	[1]
Decreases based on tax positions	(143)	[1]	(13)	[1]	(125)	[1]
Increases based on tax positions related to current period	69	[1]	17	[1]	17	[1]
Changes related to settlements with taxing authorities		[1]	0		(10)	[1]
Decreases related to the lapse of statute of limitations	1	[1]	(1)	[1]	(2)	[1]
Unrecognized tax benefits	90	[1]	156	[1]	148	[1]	48
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	54	[1]	1	[1]	1	[1]
Union Electric Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	136		124		164		15
Increases based on tax positions	0		4		15
Decreases based on tax positions	(122)		(7)		(63)
Increases based on tax positions related to current period	16		15		13
Changes related to settlements with taxing authorities			0		(5)
Decreases related to the lapse of statute of limitations	1
Unrecognized tax benefits	31		136		124		15
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	3		3		1
Ameren Illinois Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	13		11		56
Increases based on tax positions	2				0
Decreases based on tax positions	(16)		(1)		(41)
Increases based on tax positions related to current period	0		3		0
Changes related to settlements with taxing authorities			0		(4)
Decreases related to the lapse of statute of limitations
Unrecognized tax benefits	(1)		13		11
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	0		(1)
Other Affiliates [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	7		13		26
Increases based on tax positions	5		1		7
Decreases based on tax positions	(5)		(5)		(21)
Increases based on tax positions related to current period	53	[2]	(1)		4
Changes related to settlements with taxing authorities			0		(1)
Decreases related to the lapse of statute of limitations			(1)		(2)
Unrecognized tax benefits	60		7		13
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	$ 51	[2]	$ (1)

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] Primarily due to tax positions relating to the New AER divestiture. The income statement impact of this unrecognized tax benefit was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.